The present invention relates to an abnormality detecting apparatus, an abnormality detecting method, and an abnormality detecting program for a vibration-type angular velocity sensor that detects an abnormal condition of the vibration-type angular velocity sensor, and also related to a vehicle control system incorporating the abnormality detecting apparatus.
A conventional vehicle control system using an angular velocity sensor to maintain the vehicle in a normal condition is, for example, a vehicle stability control system that optimizes the brake force or torque of respective wheels in response to detection of side slipping of the vehicle or a 4-wheel steering angle control system that controls the steering angle of rear and/or front wheels of the vehicle.
This kind of conventional control system requires a yaw rate signal representing the side slipping of the vehicle as an abnormal condition of the vehicle. The angular velocity sensor produces the yaw rate signal. When the yaw rate signal is abnormal, there is the possibility that the vehicle may cause unpredictable behavior and accordingly the traveling characteristics of the vehicle become unstable.
To solve the above problem, the Japanese Patent No. 2504233 proposes the following technique (refer to page 2, right column, line 45 to page 3, left column, line 46 and FIG. 1).
For example, if a traveling vehicle rides on a relatively large pebble lying on a road, the vehicle will be subjected to a large shock. In such a case, the angular velocity sensor installed in the vehicle will receive a large shock, too. The processing circuit of the angular velocity sensor will saturate in signal processing due to this shock and, as a result, will produce a signal different or deviated far from the inherent yaw rate signal of the vehicle.
In view of the above, the Japanese Patent No. 2504233 checks whether or not the processing circuit signal of the angular velocity sensor exceeds a predetermined level, and then notifies the abnormality of the angular velocity sensor to the vehicle control system when the processing circuit signal exceeds the predetermined level. Thus, the overall control system can be stabilized.
Hereinafter, the conventional abnormality sensing system disclosed in the Japanese Patent No. 2504233 will be explained with reference to FIG. 6. FIG. 6 is a circuit diagram showing a conventional angular velocity sensor and peripheral circuit arrangement disclosed in this prior art.
When an excessive shock is applied on an angular velocity sensor 501, the angular velocity sensor 501 vibrates and accordingly an angular velocity, i.e., a yaw rate (a rotating speed of the vehicle about a vertical axis) is generated. Two piezoelectric sensing elements 506 and 507 detect this yaw rate and generate a yaw rate signal.
In a case that an excessive shock or acceleration is input, the piezoelectric sensing elements 506 and 507 produce a signal having a higher peak value compared with an ordinary signal level. As a result, an AC amplifier 512 produces an output voltage V1 having an abnormal peak value. The judging means 516 including two window comparators 516a and 516b makes a judgment as to whether or not the output voltage V1 is within a predetermined range, and outputs a READY signal V4 representing the judgment result.
However, the specific occasion that the produced signal is different or deviates far from the inherent vehicle yaw rate is not limited to the application of such an excessive shock. For example, the angular velocity sensor may receive the vibration containing the components of a driving system resonance frequency fd of a vibrator element placed in the vibration-type angular velocity sensor, its odd-number harmonics 3fd, 5fd, - - - , and a difference frequency (Δf=|fd−fs|) between the driving system resonance frequency fd and a sensing system resonance frequency fs. In such cases, the angular velocity sensor possibly produces a signal different or deviated far from the inherent vehicle yaw rate. Especially, in a case that the angular velocity sensor receives the vibration including a frequency component corresponding to Δf in the case that fd or a mechanical Q value of the sensing system is high, there is the higher possibility that the angular velocity sensor may produce an abnormality signal even when the vibration level is low.
This is believed that the processing circuit of the angular velocity sensor cannot discriminate between an actual angular velocity signal and a signal derived from the vibration of the above-described frequency component. According, for example, as disclosed in the Japanese Patent No. 3037774, it is possible to enhance or optimize the vibration-proof structure of a vibrator element placed in the angular velocity sensor so that the vibration containing a frequency band causing an abnormality signal cannot be input into the angular velocity sensor (refer to paragraphs [0007] and [0008] and FIG. 1).
However, if it is required to consider the possibility that the angular velocity sensor may further receive the vibration having a wide frequency band causing the abnormality signal or the possibility that the vibration-proof structure may deteriorate some day, the above-described conventional technique will be insufficient in that the angular velocity sensor may be adversely influenced by the frequency band causing an erroneous output.